Beacon radars have been in extensive use for ATC applications. The role of beacon radar in all its forms is increasing as more and more services are provided through the bi-directional link available between the ground stations and the aircraft. Known prior art include beacon radars with mechanically rotating antennas, wide area multilateration systems and TCAS.
The earliest form of a beacon radar consisted of a directional, mechanically rotating antenna and a beacon transmitter/receiver (transceiver) connected to it. The transceiver sends interrogations at a frequency of 1030 MHz through the antenna and receives replies from the targets. The range of the target is determined by measuring the time between transmission and reception and subtracting the internal transponder delay. The net round trip delay (RTD) multiplied by the speed of light is twice the target range. Target azimuth is determined from the known azimuth of the spinning antenna. Beacon radars with scanning antennas require mechanical support structures that are heavy and not easy to transport. Further, the size of the antenna is directly related to the desired azimuth accuracy. Thus, if the azimuth accuracy requirement is high for the beacon radar, the antenna size is larger, making it heavier and more difficult to transport.
Beacon radars with scanning antennas sweep the detection volume at a constant rate that is equal to their rotation rate. In air traffic control (ATC) applications, more and more of the services depend on unsolicited transmissions (ADS-B Squits) from the airborne transponders. In order to receive all unsolicited transmissions the receiving system needs to be open in all azimuths at all times. Only under these conditions will the probability of intercept (POI) of target squits be 100%. Beacon radars using scanning antenna are unsuitable for these services because the unsolicited transponder transmissions is totally asynchronous with the antennas rotation.
Multilateration systems are based on multiple receivers, each measuring high accuracy time of arrival of all replies. The TOA (Time of Arrival) data is processed in a central processor. It can be shown that Differential Time of Arrival (DTOA) from, at least, 3 stations are sufficient to find 2D target position and at least 4 stations are required to find 3D target position. Multilateration systems can also use transmission and RTD to help the localization process. In particular, this capability is useful when the targets are outside of the baseline of the multilateration receive stations.
Multilateration systems overcome the POI issue of target squits associated with the scanning antenna systems because the receivers are open at all times in all azimuths. Therefore, multilateration systems can support ADSB and other services on the bi-directional link to the aircraft, and can also support ADSB verification because, by their nature, they estimate target position, or at least hyperbolas where the target can be present independent of the ADSB report. However, multilateration systems require relatively large baselines (distance between receiving stations) for high accuracy. As an example, in order to obtain accuracy comparable to BI-6, the baseline has to be in the order of 2000 meters (BI-6 is the Air Traffic Beacon Interrogator 6, which is a high performance air traffic beacon radar based on a large rotating antenna). Multilateration systems also require data links between the individual stations and the central processor, which add to the cost and complexity of the system. TCAS systems provide instantaneous hemisphere coverage with azimuth estimation. These systems typically estimate azimuth by comparing the amplitude and/or phase between adjacent antenna quadrants. The inter-element spacing between antenna elements is in the order of half a wavelength and, therefore, there are no ambiguities associated with phase comparison azimuth estimation methods. However, the size of the antenna is limited and, therefore, the accuracy is about 10 to 20 times worse than BI-6. TCAS antennas and azimuth estimation could be used for beacon surveillance systems, extracting range from RTD. Such systems have 100% POI and can, thus, support ADSB and other services if the antenna is connected to a transceiver/processor that support such services.
Interferometry can be used to obtain very high azimuth accuracy with much smaller baselines compared to multilateration. However, when the spacing between interferometric elements is over half of the wavelength rollover of 360 degrees of phase occurs and the measurement becomes ambiguous.
Further, where indirect reflections of a transmitted signal from other objects (i.e., multipath) mix with the transmitted signal coming directly from the target, the received signal is distorted by the reflected signals. The distortion of the received signal creates ambiguity that results in angle of arrival estimation error.
Existing methods for resolving this ambiguity is to add additional baselines, which requires additional antennas and receivers at different physical locations. This adds significant cost and complexity to any system. Prior art regarding adaptive techniques to mitigate signal distortion caused by multipath mention the possibility of adaptive nulling in space, using the antenna array, an optimal set of complex weights is calculated and the complex output from each antenna is multiplied by these weights and summed. However these techniques an adaptive null in space towards multipath sources only if the multipath signals can be separated from the direct path signals.
What is needed is a system and method for determining the angle of arrival of a received signal with high accuracy that can resolve the interferometric ambiguity and determine the angle of arrival correctly without having to add additional antennas and receivers at different physical locations.